EVALUATION OF THE METHOD OF COLLECTING SUSPENDED SEDIMENT FROM LARGE RIVERS BY DISCHARGE-WEIGHTED PUMPING AND SEPARATION BY CONTINUOUS-FLOWCENTRIFUGATION

A method for collecting suspended sediment samples has been developed that pumps a discharge-weighted volume of water from fixed depths at f our to 40 locations across a river and separates the suspended sedimen t in the sample using a continuous-flow centrifuge. The efficacy of th e method is evaluated by comparing the particle size distributions of sediment collected by the discharge-weighted pumping method with the p article size distributions of sediment collected by depth integration and separated by gravitational settling. The pumping method was found to undersample the suspended sand sized particles (> 63 mum) but to co llect a representative sample of the suspended silt and clay sized par ticles (< 63 mum). The centrifuge separated the silt and clay sized pa rticles (< 63 mum) into three fractions. Based on the average results of processing 17 samples from the Mississippi River and several of its large tributaries in 1990, about 10% of the silt and clay sized mater ial was trapped in a centrifuge bowl-bottom sealing unit containing th e nozzle and consisted of mostly medium and coarse silt from 16 to 63 mum. About 74% was retained on a Teflon liner in the centrifuge bowl a nd consisted of sizes from 0.1 to 63 mum. About 9% was discharged from the centrifuge in the effluent and was finer than 0.1 mum. About 7% w as lost during the processes of removing the wet sediment fractions fr om the centrifuge, drying and weighting. The success of the discharge- weighted pumping method depends on how homogeneously the silt and clay sized particles (< 63 mum) are distributed in the vertical direction in the river. The degree of homogeneity depends on the composition and degree of aggregation of the suspended sediment particles.